The invention relates to a process and an arrangement for the automatic focusing in a photographic enlarging or copying apparatus with a variable enlarging scale.
Known enlarging apparatuses comprise a plane for receiving the copying material, particularly photographic paper; a lens system support for supporting a selected enlarging lens; as well as a frame holding means for receiving a film original to be copied. The lens system support and the frame holding means are parts of a projecting head by means of which the film original is projected onto the copying material. In order that the lens draws a sharp image of the original in the copying plane, the following well-known relationships must be satisfied for the distances between the level of the lens and the level of the original and between the level of the lens and the copying level: ##EQU1## wherein a is the distance between the lens level and the level of the original; b is the distance between the lens level and the copying level; c is the distance between the copying level and the level of the original; m is the enlargement ratio; and f is the focal length of the used lens.
In the case of known enlarging apparatuses, the copying level is generally fixed, whereas the frame holding means and the lens system support are movable and can be adjusted in the direction of the optical axis of the lens. The setting of a desired enlarging ratio is made by adjusting the position of the projecting head of the enlarging apparatus relative to the fixed copying level; i.e., the distance c between the level of the original and the copying level is changed. For the focusing of the image corresponding to a set enlarging ratio, the lens must then be moved into the position for which the relationships (1) are met. For an automatic focusing device, the necessity therefore arises of determining for each setting the absolute position of the frame holding means as well as of the lens system support.
Electronic devices for the acquiring of such absolute position values are, for example, so-called digital rotary encoders, where a coding disk is coupled with the part whose position is to be acquired, and the position of the movable part is encoded by the angular position of the coding disk. If a high measuring resolution is required, the measuring and setting range of this system is limited, because the coding disk can contain only a limited number of codes. In any case, the manufacturing costs in the case of the application discussed here, where a high resolution is required over a relative large area, would be inadmissibly high.
From German patent publication No. DE-A 28 05 030, a process and an arrangement are known for the automatic focusing of a reproduction camera, where, for the detection of positions, incremental rotary encoders are provided. In this case, a sequence of electric pulses is generated by the movement of a rotary disk, and the distance to be moved, starting from a certain initial position, is determined by the counting of the emitted pulses. In order to keep the influence of possible errors during the counting of the occurring pulses as low as possible, and to obtain absolute position values, it is provided that the frame holding means and the lens system support means, at the start of each setting operation, are each moved into a reference position, to which a specific count value is assigned that characterizes the distance from the copying level. When a reference position is reached, the pertaining counting device, irrespective of the reached count, is reset to the counting value that is fixedly assigned to the reference position, whereby a reference value is set. After the selection of a desired enlarging ratio m and with the known focal length f of the lens, the focusing is then performed by calculating with a computing device the distance a and b on the basis of the above-mentioned relationships (1), and by using a motor-driven setting device move the frame holding means and the lens system support into the corresponding positions.
Although the use of an incremental measuring device is advantageous with respect to the costs and the achievable resolution, the fixed reference points on the moving paths of the frame and of the lens result in increased expenditures for detectors, such as microswitches or the like, for detecting these reference positions. The requirement of the precise positioning of the detectors also entails the risk of a possible instability in the course of time. Finally, the indicated process requires the knowledge or a preceding determination of the focal length of the used lens because it is based on the result of the calculation derived from the relationships (1).